Cargo limit question

Perhaps this has been asked before.  I notice the cargo limit on the 180 is ~997lbs, and a full tank of water is around 450lbs or something.  That gives you only ~500lbs of cargo to load.    That doesn't seem like much considering dishes in cabinets, clothes, food in fridge, our bodies, etc.  Seems like we would be border line max capacity sitting at the campsite.  I understand that it recommended to empty water tanks before driving.  I also see people adding additional propane tanks, more batteries, decorations.  Is it just more of a concern when driving, and sitting stationary isn't a big deal? Would hate to bend my axle.  I seen some people upgraded theirs to 5000lbs. I guess the question is, am i unnecessarily worrying?

A full tank of water (30 gallons) is only 250lbs.  Then the hot water heater holds 6 gallons thats another 50lbs.  (Water is 8.3lbs per gallon)

Nope you're right ^^^. 10-15% of the weight should be on the tongue, not the axle. I don't tow with a full tank of fresh water ever, but fill at the nearest available supply to my final destination. There is no real good reason to pull 30 gallons of water cross country. If you are dry camping, 30 gallons isn't going to last you long enough to justify pulling it 500 miles anyway. At least in my opinion. What are you trying to do???